Pineapple leaves are an abundant and underutilized biomass resource released from cutting old plants with low productivity. The renewable waste was, therefore, used for the preparation of the Fe3O4/C composite via single-stage pyrolysis with FeCl3 addition. The results revealed that FeCl3-activation produced Fe3O4 crystals with an estimated content of 44 wt% in the resulting Fe3O4/C composite. In addition, the specific saturation magnetization of Fe3O4/C reached 4.62 emu/g, which supported its magnetic separatability. The material was accordingly explored as an oxidation catalyst for treating methyl orange (50 ppm) at pH 3.0. As a result, Fe3O4/C partly removed methyl orange with an adsorption capacity of 9.9±2.4 mg/g. When H2O2 (300 ppm) was added, 0.40 g/L of Fe3O4/C decolorized 93±2% methyl orange with an average rate of 72±3 mg.g−1.h−1. Furthermore, this decolorization obeyed pseudo-first-order kinetics with a rate constant of 0.030 min−1. Overall, the findings from this study strongly advocate for the use of low-cost Fe3O4/C as a highly effective catalyst for methyl orange removal from wastewater. Moreover, these results pave the way for further exploration of pineapple leaf-derived Fe3O4/C for a wide range of potential applications, contributing to sustainable waste management and green chemistry practices.